The preschool years mark a critical period for the emergence of developmental psychopathology, including symptom dimensions consistent with ADHD. However, the availability of assessment tools to effectively detect such mental health problems early in childhood is limited, in part because the early precursors and neural bases are not well understood. The preschool period is characterized by rapid neurocognitive development reflected in young children's executive control (EC) capabilities, and individual differences in EC figure prominently into mental health disorder and functional impairment, making it a potentially important candidate to target for early assessment/intervention. Recent developmental models of EC suggest that executive task performance in younger preschoolers is driven primarily by bottom-up stimulus/response processing before a qualitative shift occurs around age four, after which top-down executive processes begin to contribute to behavioral performance. It is unknown what the precise neurophysiological mechanisms of these complimentary processes are before and after this critical transition, and how they might differ in children at risk for metal health problems. The long-term goal is to explicate the neurophysiological mechanisms underlying these processes across this developmental transition and understand how they relate to risk for child psychopathology. The objective of the proposed research is to characterize distinct neurophysiological signatures of the respective mechanisms in typically developing children before and after the transition, and to identify differences in these signatures in children displaying ADHD symptoms after the transition. The rationale is that this knowledge will inform how these mechanisms function at different developmental stages and in a clinical sample to lay the groundwork for future longitudinal study and intervention. The central hypothesis is that distinct neurophysiological signatures of bottom-up stimulus/ response processing will be identifiable before and after the transition, but neural activity related to to-down EC will only be observed at later ages and further be diminished in children with ADHD symptoms. The specific aims of the research are to: 1) Characterize the neurophysiological signatures of bottom-up and top-down processes in younger and older preschoolers; 2) Determine the relative power of the neurophysiological signatures reflecting both bottom-up and top-down processes in predicting specific and general ADHD symptom dimensions in 3.5- and 5-year-olds. The project is innovative in its objective to measure brain activity related to EC and ADHD risk in this critical age range, and to do so within the framework of sophisticated statistical models which form the basis of the hypotheses. The significance of this research is that it will explicate the mechanisms underlying the qualitative shift in preschool EC development and identify how individual differences in these mechanisms correspond to risk for child psychopathology. This work will clarify the importance of EC as a critical target for early assessment/intervention and ultimately enhance national mental health.